The wireless sensor network (WSN) is a wireless network formed ad hoc by great quantities of sensor nodes with low complexity, and each sensor node in the network includes a sensing module, a processing module, a communicating module, and a power module to carry out the three basic functions of data collection, data reception/transmission, and data forwarding. The wireless sensor network has the characteristics of high reliability, easy deployment and scalability. The advent of new-generation, smaller, cheaper, and low-power-consuming devices, the enhancement in data calculation and processing capability brought about by distributional calculation, and the development of Micro-Electro-Mechanical Systems (MEMS) together make it possible to develop multifunctional sensors with low production cost, low power consumption, small size, and short-distance communication capability, thus laying the ground for the generation and development of the wireless sensor network. The wireless sensor network does not require the support of fixed networks, is of characteristic of rapid deployment and strong resistance to damage, can be widely applied to such fields as the military, industry, construction, goods storage, smart housing, and environment protection, and therefore draws widespread attention.
Medium Access Control (MAC) protocol decides the mode whereby a radio channel is used, to allocate limited radio communication resources among sensor nodes. Power saving and high efficiency are important objectives in the study of MAC layer protocols of the wireless sensor network. IEEE 802.15.4 Standard is proposed by the IEEE Association of Standardization with respect to a Low Rate Wireless Personal Area Network (LR-WPAN), and is currently one of the most important protocols for the wireless sensor network. Beacon Management and Channel Access Control are two important functions in an IEEE 802.15.4 MAC sub-layer. Two communication modes, i.e., Beacon Enabled Communication and Beacon Disabled Communication, are provided for selection in LR-WPAN.
IEEE 802.15.4 Standard divides all wireless devices in networks into two classes, namely Full Function Devices (FFD) and Reduced Function Devices (RFD), of which the former can carry out all functions of the protocol, and the latter can only carry out some simple functions. Moreover, these devices are functionally divided into three types of nodes, namely a PAN coordinator, coordinators and common nodes (devices), of which the PAN coordinator is the master node of the entire network, and there may be only one PAN coordinator in a single IEEE 802.15.4 network; the coordinator usually achieves synchronization with surrounding nodes by sending beacons, and has the function of forwarding packets; and the common node only has simple transceiving functions and cannot forward packets. An FFD can function as a PAN coordinator, a coordinator or a common node, whereas a RFD can only function as a common node.
A peer-to-peer network is a key topological structure supported by the IEEE 802.15.4 Standard. When an effective coordinator is present in a peer-to-peer network, any two random nodes within the range of communication can communicate with each other.
In a beacon enabled network, a sink node periodically broadcasts a beacon frame. The beacon frame indicates the beginning of a superframe. Communication among sub-nodes makes use of time-slot-based CSMA/CA channel access mechanism, in which, whenever a sub-node needs to transmit a data frame or a command frame, it firstly positions the boundary of the next time slot, and then waits for a random number of time slots. After the waiting, the sub-node begins to detect the channel status—if the channel is idle, the sub-node begins to transmit data at the boundary of the next usable time slot, and if the channel is busy, the sub-node needs to wait again for a random number of time slots, check the channel status, and repeat the process until an idle channel appears.
In a beacon disabled communication network, the sink node does not transmit a beacon frame, and each sub-node makes use of a CSMA/CA mechanism, which is not divided by time slots, to access the channel. Communication process according to this mechanism is as follows: whenever a sub-node needs to transmit data or an MAC command, it firstly waits for a random period of time, and then begins to detect the channel status—if the channel is idle, the sub-node immediately begins to transmit data, and if the channel is busy, the sub-node needs to repeat the above processes of waiting for a random period of time and detecting channel status until it is possible to transmit data.
In the wireless sensor network, a node, particularly such as a coordinator and a common node, is usually powered by a battery with extremely limited capacity, and charging or replacement of the battery is often inconvenient or even impossible, thus leading to failure of the node in the wireless sensor network, and resulting in the breakdown of the entire network.
Accordingly, it is a key task in the study of the wireless sensor network as how to save power consumption of network nodes in the process of use of the wireless sensor network to hence elongate the life thereof.
In the traditional study of saving energy efficiency of the wireless sensor network, the conventional OSI layered protocol model of an IP network was usually employed. The model simplifies the complex network design, lowers the production cost, and enhances such performances as execution efficiency, but optimization thereof is usually carried out at a specific network layer, so that improvement over the network performance is relatively limited, and enhancement of energy efficiency is usually achieved at the expense of network delay performance. Presence of this problem restricts the wireless sensor network from being developed for the more potential real-time services. To break the bounds of traditional concept of hierarchy, to take as objective the optimization of network performance, to perform cross-layer optimization by unifying each layer of the network, and to achieve seamless interaction of information amongst each of the layers are progressively meaningful to guarantee network quality of service (QoS) and improve the overall performance of the network.
As a result, there is very wide room for application to research for excellent cross-layer optimization technology in wireless network to maximize the life of the network while guaranteeing time effectiveness of data.
For example, both Chinese Patent Applications No. 200810056455.X and 200710049792. 1 have proposed methods for power consumption of wireless sensor networks based on cross-layer designs, and both of these employ the power saving mode of lowering network power consumption by controlling the transmission power of the node.
Adaptive regulation of network node power is a common concept in the current study of cross-layer power saving of the wireless sensor network, but as seen from the currently available wireless sensor network products, power regulation of sensor nodes at the physical layer requires a long initialization process, and is not applicable to the circumstances of frequent changes in network topological structure and data traffic.
Moreover, when other wireless communication networks are concerned, if there is restriction on energy, the problem of saving power for the network should also be considered.